Why IQ Fluctuates Over Your Lifespan

In 1932, the entire population of Scottish 11-year-olds (87, 498 children) took an IQ test. Over 60 years later, psychologists Ian Deary and Lawrence Whalley tracked down about 500 of them and gave them the same test to take again.

Turns out, the correlation was strikingly high -- .66, to be exact. Those who were at the top of the pack at age 11 also tended to be at the top of the pack at age 80, and those who were at the bottom also tended to stay at the bottom. Equally as interesting, the correlation was far from perfect. A few outliers could be found. One person had an IQ of over 100 at age 11, but scored just over 60 at age 80. There are many possible reasons for this outlier, including dementia. Other folks showed IQ increases as they aged. On average, people's individual (or absolute) scores on the test taken again at age 80 was much higher (over one standard deviation) than their scores had been at age 11, even though the rank ordering among people stayed roughly the same.

These results are illustrative of what psychologists find over and over again. IQ tends to remain relatively stable over the lifespan. The key word here is relative. IQ researchers are interested in explaining differences. Developmentally speaking, an individual's intelligence is not fixed at birth. Although rank ordering of scores tends to remain stable (relative change), scores within individuals fluctuate quite a bit (absolute change). As the noted IQ researcher (and my mentor) Nicholas J. Mackintosh notes in his superb book "IQ and Human Intelligence,"

"If, aged 40, you obtain the same absolute score on the same test as the score you had obtained at age 10, something would have gone seriously wrong in your life. The long-term stability of IQ means only that your standing relative to others of your age stays much the same."

In childhood and adolescence, intelligence is particularly vulnerable to plasticity and change. The relation between brain size and IQ is lower in children than in adults. IQ is related to brain development in complex ways. One study followed 300 children up to early adulthood. At age 7, the higher IQ children (greater than 120) tended to have less cortical thickness. Soon after, however, the high IQ children showed a rapid increase in cortical thickness, overtaking the other children and peaking at age 11-12 before slowly declining to about the same level as the others. The researchers concluded

"'Brainy' children are not cleverer solely by virtue of having more or less grey matter at any one age. Rather, intelligence is related to dynamic properties of cortical maturation."

Of course, the relation between genes and cortical maturation need not be direct. Higher IQ children may act differently in the world and elicit different reactions from others, which in turn sculpts their brains. It has long been known that experience influences cortical thickness. Regardless of the causes of these findings, these results are fascinating and demonstrate the plasticity of human intelligence, especially among adolescence. A recent study which has been garnering a lot of media attention further illustrates this point.

Sue Ramsden at University College London and her colleagues took a batch of 33 healthy adolescents (aged 12-16), with a wide mix of abilities. She tested their IQ in 2004 and then again 3-4 years later. During each test session, each child had their brains scanned using fMRI. Neither the participants nor their parents had a clue that they would be tested again. Consistent with earlier research, scores changed quite a bit from one testing session to the next. Thirty-three percent of the participants showed a clear change in their total IQ score. To highlight the range: One participant showed an increase in IQ score by 21 points, whereas another showed a decrease of 18 points. These are dramatic changes!

The researchers weren't just interested in test score changes, but were also interested in whether the changes were meaningfully reflected in brain structure. If they could link their changes to brain structure, then it would suggest these IQ fluctuations aren't merely the result of "measurement error." Indeed, they found that changes in the verbal portions of the IQ test (e.g., vocabulary, verbal comprehension) were related to changes in grey matter density and volume in the left motor cortex; a region that is known to be activated by the articulation of speech. Changes in the non-verbal portions of the IQ test (putting blocks and pictures together in their proper arrangement, completing pictures appropriately) were related to to grey matter density in the anterior cerebellum; a region that is known to be activated by motor movements of the hand. Interestingly, the grey matter changes that were associated with verbal and non-verbal IQ changes were not associated with regions normally found to be associated with IQ (frontal and parietal regions). Their results suggest that particular forms of intelligence (but not overall IQ scores) are reliant, at least in part, on sensorimotor skills and that such fluctuations aren't purely the result of 'measurement error'. According to the researchers,

"The implications of our present findings is that an individual's strengths and weaknesses in skills relevant to education and employment are still emerging or changing in the teenage years."

I have a few quibbles. It was a small sample, it's not clear that the tests they administered were age-appropriate for all of their participants, and some of the high-IQ children tested at the first time point may have found the test too easy (showing 'ceiling effects'). Future research should look at larger samples, using a number of different tests, on a wider range of ages. It remains to be seen if the same levels of plasticity are evident among older populations.

At any rate, it's a neat little study and contrary to an article in the Guardian, which stated that the study "contradicts a long-standing view of intelligence as fixed", the study is actually quite consistent with what IQ researchers have been finding for over 50 years! No sensible IQ researcher would say that intelligence is fixed. Anyone who does tell you that is a nut job.

In fact, leading IQ test makers fully acknowledge that people don't have "an" IQ. That's why they are increasingly moving away from a focus on total IQ scores to observing how children behave on the different sections of the test. As Alan S. Kaufman (no relation), an IQ test pioneer, notes in his excellent book "IQ Testing 101":

"Ultimately, IQ tests are not about numbers; they're about people ... People who are given an IQ test are evaluated for a reason. No one has ever been referred for testing because of terminal niceness or because someone said, 'This person is so incredibly normal -- you've just got to test her.' When there's a test, there's usually a real-life question to be answered, sometimes a dilemma, which demands attention and intervention ... Whatever the nature of the problem, it can't be solved by computing a global IQ. An intelligent, aware, knowledgable clinician is an even more important tool than the test itself -- an astute observer who treats the global IQs as often the least important outcomes of the evaluation."

"The important lessons here are that IQ tests are built based on clinical observations, they offer far more to the skilled examiner than a bunch of test scores, and a psychologist's experience with a test is an invaluable commodity that cannot be overlooked."

The bottom line here is that intelligence was never, and will never, be fixed at birth. Be very skeptical of any media report that argues that a new study overturns research showing that it is. The development of intelligence is one of the most fascinating, and important, research topics in psychology (at least, I think so!). Every child enters the testing session for a reason, and every child has his or her own unique constellation of traits and life experiences. The key is not to determine that child's one "true" IQ score, but to discover as much as possible about that child in order to help him or her flourish.